Photolithography of SU-8 microtowers for a 100-turn, 3-D toroidal microinductor
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We present a photolithography scheme for ultra-tall, high-aspect-ratio microstructures. While increased height of microstructures can expand the design capability of various microdevices, it has been challenging to achieve the ultra-tall microstructure, 1 mm or higher, using a well-known negative photoresist, SU-8. One of the reasons is the high absorption rate of 365-nm ultra-violet light during the exposure process, although it used to be recommended for the SU-8 process. We report on optical characteristics of microlithography, in particular the 365- and 405-nm wavelengths, and present the lithography method for ultra-tall micropillars with a height of 1 mm or higher, called microtowers. While the 365-nm wavelength is experimentally validated with its high attenuation inside the SU-8, higher transparency of the 405-nm wavelength with a thicker SU-8 is reported to be suitable for ultra-tall micropillar structures. Assuming exposure time causes the color change of the SU-8, transparency of the SU-8 as a function of exposure time is measured with a thick SU-8. SU-8 microtowers with various heights are reported, including an array of 2000-µm-tall microtowers and a state-of-the-art 7000-µm microtower. To demonstrate usefulness of the proposed fabrication method, an array of 1000-µm-tall microtowers are successfully fabricated to form a 100-turn, 3-D toroid inductor. The fabricated inductor shows average inductance of 950 nH in the frequency range of 0.1 to 10 MHz, a low-frequency resistance of 5.4 Ω at 0.1 MHz, and a quality factor of 22 at 60 MHz.
KeywordsMicrotower 3-D toroid inductor High-aspect ratio SU-8
An epoxy-based negative photoresist, SU-8, has become widespread in microelectronics, and bio, optics, and radio-frequency (RF) research [1, 2, 3, 4, 5, 6]. Relatively easy to use, SU-8 is chemically stable and well-known for its mechanical characteristics . Moreover, high-aspect-ratio structures in the hundredths micron scale have demonstrated how various microdevices keep the overall dimension of the microdevices small by effectively utilizing conventionally unused space. For example, an SU-8, pillar-framed metalized monopole antenna holds a small footprint area compared to a conventional 2-D printed design on a circuit board [8, 9]; while the expanded height of the antenna allows for more extensive frequency selection. We used an array of SU-8 pillars for micromachined 3-D toroid inductors [10, 11, 12, 13]. For these microinductors, we used 600- to 1000-µm support materials, including an SU-8 pillar array. However, performance of the antenna or the inductor could be expanded with a taller height and higher-aspect ratio of the SU-8 structures.
In this paper, a fabrication method of microtower structures is presented using the mercury-vapor lamp, UV lithography system. To selectively choose the UV wavelength, an acrylic plate was adopted as an optical filter, which passed the UV wavelength of approximately 380 nm or longer [17, 18]. The attenuation of 405-nm UV light as a function of SU-8 thickness was explored. Also, the transparency variation of the 405-nm UV light as a function of exposure time was reported. Microtower arrays, with multiple heights from 1000 to 7000 µm, were fabricated with 405-nm exposure and compared to similar microtower structures with broadband UV exposure. We also demonstrated inclined 3-D microstructures using multidirectional UV lithography [19, 20]. The 3-D toroidal microinductors utilized the microtower structures at vertical winding as demonstrated. An array of 1000-µm-tall, SU-8 microtowers was implemented for a 100-turn, 3-D microinductor. The fabricated inductor was electrically characterized resulting in an average inductance of 950 nH in the frequency range of 0.1 to 10 MHz.
Optical characteristics of UV lithography
Results and applications
A fabrication scheme of a high-aspect-ratio, tall microstructure in the height range of 1000 to 7000 µm was introduced. A microstructure with high-aspect-ratio and taller than 1000 µm is named as a microtower, since fabrication of the structure requires a narrow-band single-peak wavelength during the exposure. A spectrum of the UV exposure with an acrylic filter was verified to eliminate the i-line and showed approximately 60% transparency to the h-lines. Since the SU-8 was designed to be crosslinked under UV range, the wavelength region at the h-line was assumed to be reactant to the SU-8. To prove the concept, an array of 2000-µm-tall microtowers were successfully fabricated with a smooth straight sidewall, while a 1000-µm-tall microtower array showed wavy sidewalls without using the acrylic filter during UV exposure. As an alternative 3-D UV exposure scheme, a 1500-µm-long inclined microtower was successfully demonstrated, validating the batch fabrication as well as the process compatibility with multidirectional UV lithography. A 100-turn toroidal microinductor was fabricated and electrically characterized. Vertical windings were fabricated based on the proposed fabrication scheme with 1000-µm height. An average inductance of 950 nH in the frequency range of 0.1–10 MHz was observed, with a low-frequency resistance of 5.4 Ω and a peak quality factor of 22 at 60 MHz.
JK carried out most experiments and drafted the manuscript. HAT and MA conducted and analyzed the light transparency and intensity attenuation data. All the authors discussed the proposed method and experimental results. All authors read and approved the final manuscript.
The author thanks Dr. Mark G. Allen at the University of Pennsylvania for valuable advice and support and Ms. Fahmida Shiba for the 4-mm tall SU-8 structure.
The authors declare that they have no competing interests.
This work has been partially supported by an international collaborative research project sponsored by the Korea Institute for Advancement of Technology (KIAT, 2017-52, N058900002) and Samil Tech Co. LTD.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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